1. Field of the Invention:
The present invention relates to an apparatus for measuring the comprehensive state of vector displacement between a group of at least two pairs of longitudinally spaced side-looking radar arrays on a flexible host vehicle.
2. Description of the Prior Art:
Distributed antenna arrays as shown in FIG. 1 refer to the application of several arrays on various portions of the same host vehicle. These various arrays could be either conformal in architecture or planar behind conformal radomes. One of the advantages of a distributed array concept over the more conventional single antenna array is the ease of integration into the host vehicle. This is due to the increased flexibility created by the placement of multiple small arrays instead of the intergration of a single large antenna. Another potential advantage of distributed arrays is the possibility of increased radar coverage for the host vehicle. FIG. 2 is an example of the conventional approach to the radar antenna on a host vehicle. The example here is the AWACS airplane with a non-conformal radar antenna located within a radome located at the top of the aircraft. However, this utilization of a single antenna not fully integrated into the body of the airplane results in increased radar cross-section and increased drag resulting in a decrease of "time on station". The term "time on station" refers to the amount of time that the aircraft can cruise on its surveillance mission. A greater amount of drag is experienced by the airplane with a single radar antenna not fully integrated into the body of the airplane. Increased drag decreases the amount of time that the airplane can be in the air performing its radar scanning function.
Each of the arrays for a distributed array concept as described in this application are either conformal which means they conform to the outside skin of the host vehicle in nature, or are planar in nature behind conformal radomes. It is necessary, however, that each of the arrays be accurately located relative to an inertial frame of reference. This accuracy of loction is required because relative motion exists between each of the antennas due to structural flexure experienced by portions of the same host vehicle during flight.
The problem to be solved then is a mechanism or apparatus that will measure three linear translational motions, and three rotational motions for each of three groups of distributed antenna arrays. The three linear translational motions in a Newtonian coordinate system are traditionally designated as X, Y and Z. The three rotational motions in a Newtonian system are usually characterized by azimuth, elevation and roll. We will utilize the Newtonian refernce coordinate system in this application where X, Y and Z will define the three linear translational motions; and azimuth will refer to rotation about the Z axis, elevation will refer to rotation about the Y axis, and roll or R will refer to rotation about the X axis. The utilization of an antenna array system distributed about a host vehicle is also be applicable to an aircraft carrier or any other larger vehicular means wherein flexure or structural changes occur during the operation of the host vehicle impacting the relative positions of the radar antenna arrays. Generally speaking, six degree of freedom motion will not be experienced by all the arrays on a particular host vehicle because there will be indigenous structural constraints of the aircraft vessel that will not permit motion in all six degrees of freedom. In these instances, the apparatus as taught by this application can obviously be simplified in those situations.
A brief description of the preferred embodiment of this application is a laser/sensor alignment mechanism or apparatus having laser autocollimators or telescopes for measuring three linear translational motions and three rotational motions for each of three arrays of a distributed antennl array system on a host vehicle. This apparatus would have a first station with an alignment feature or reference, a laser source and four optical instruments utilizing the laser source by means of beam splitters and turning mirrors. These four instruments would transmit the beams to appropriate retroreflectors at a second station. The returns from these reflectors are then used to derive measurement signals in six degrees of freedom. This second station is also equipped with a similar laser and sensor package to reflect beams from retroreflectors on a third station to measure six degrees of freedom of the third station with respect to the second station.
This is a passive system as defined by the lack of correctional opportunities afforded the arrays after the determination that they are out of alignment. An active system would be defined as a gimballed system or a system that makes correctional changes in the position of the antenna arrays after the determination that there is indeed misalignment between the inertial reference point and the antenna array stations. A careful review of the prior art revealed the utilization of an active system as taught by U.S. Pat. No. 4,283,688 to Wayne B. Lloyd, et al., dated Aug. 11, 1981, entitled "Laser Auto Alignment System". In this U.S. patent, an auto alignment system for a laser having a pair of reflector assemblies capable of providing six degrees of freedom of movement to the reflector elements formed as a part of each of the reflector systems is taught. During the utilization of the system, that interconnection between the reflector unit and a gimballed knuckle joint provides for two degrees of freedom while the translational mechanism and its interconnection with the gimbal knuckle provides an additional four degrees of freedom. This auto alignment system maintains the correct relationship between the reflecting elements of the laser as well as providing a cooling mechanism thereof. This active system is distinct from the preferred embodiment of this application because of its capability of realigning the retroreflector and after its termination of misalignment. This is commonly referred to in the art as a Benchless Laser system.
The preferred embodiment of this application resolves the problem of high drag and resultant decreased time on station. It also provides a reduced radar cross-sectional area through the utilization and determination of the alignment radar antenna arrays all located upon a single host vehicle.